A Close Encounter with Jupiter

A close encounter with Jupiter

This weekend the Solar System's largest planet will
be brighter and nearer to Earth than at any time in the past
12 years.

UPDATE: The closest-ever
pictures of Jupiter's volcanic moon Io have been released today.
See IoFlyby.com
for details.

October 22, 1999: Lately, you
may have noticed a bright light in the sky that rises above the
eastern horizon around dinnertime. It's not an airplane or a
UFO -- it's Jupiter, the largest planet in the solar system.
This weekend Jupiter will be brighter and closer to the Earth
than it has been for a dozen years.

Right: Duane Hilton's rendering of
Jupiter and the full moon on October 23, 1999 as seen near 10
p.m. local time from Bryce Canyon.

On October 23, 1999 Jupiter will be "at opposition."
That's an astronomical term that simply means you can draw a
perfectly straight line between the Sun, the earth, and Jupiter
as shown below. This geometry isn't unusual; Jupiter is at opposition
about once a year. However, there is something special about
this year's opposition. Jupiter is passing through the part of
its elliptical orbit that is closest to the Sun. That means it
will also be closest to Earth. On October 23 Jupiter will be
368 million miles away from our planet -- just a hop, skip, and
a jump by cosmic standards.

Left: When Jupiter is at opposition
it is directly opposite the Sun as seen from Earth. Compared
to Earth, which zips around the sun once a year, Jupiter completes
one circuit around the Sun rather slowly, once every 12 years.
Opposition takes place about once per year as the Earth speeds
by the slower-moving giant planet.

To see Jupiter this week, simply go outside shortly after
the sun sets and look toward the east. Jupiter, blazing at magnitude
-3, will be rising above the eastern horizon. The giant planet
will be visible all night long. It crosses the southern meridian
at local midnight and sets in the western sky at dawn.

This is a good time to look at Jupiter through a telescope. Jupiter
will appear to be nearly 50 arc seconds in diameter, approximately
36 times smaller than the full moon. That's big! Even inexpensive
department store telescopes will be powerful enough to reveal
light and dark bands crossing Jupiter's equator and Jupiter's
four largest moons: Io, Europa, Ganymede and Callisto. It's also
a good time to view Jupiter's famous Great Red Spot, a gigantic
storm system that has been churning for at least 300 years. Tonight,
October 22, the Red Spot will move across the center of Jupiter's
disk, as seen from Earth, around 9:19 p.m. EDT. A six-inch or
larger telescope and good viewing conditions are recommended
for seeing the red spot. The Galilean satellites can easily be
seen through a good pair of binoculars. (As an interesting exercise,
see how steadily you can hold the binoculars as you're viewing
Jupiter. It's not as easy as it sounds!)

Left:
This Hubble Space Telescope image of Jupiter shows a hurricane
twice the size of the Earth. It has been raging at least as long
as telescopes could see it, and shows no signs of slowing. It
is Jupiter's Great Red Spot, the largest swirling storm system
in the Solar System. Like most astronomical phenomena, the Great
Red Spot was neither predicted nor immediately understood after
its discovery. Still today, details of how and why the Great
Red Spot changes its shape, size, and color remain mysterious.
[More]

If you're out this weekend admiring Jupiter, don't forget that
NASA is watching the giant planet, too, although with a better
view. NASA's Galileo spacecraft has just marked the tenth anniversary
of its launch aboard the Space Shuttle Atlantis on October 18,
1989. Galileo entered orbit around Jupiter in December 1995.
Since then it has revolutionized our understanding of Jupiter
and its moons by sending a probe into Jupiter's atmosphere, finding
new evidence for water on Europa, and most recently zipping a
mere 600 km above the volcanoes of Io. Many of Galileo's impressive
accomplishments are described at JPL's new Galileo home page:
http://galileo.jpl.nasa.gov.
The current Galileo Extended Mission will conclude later this
year with a daring pass less than 300 km above Io, possibly flying
through the plume of a sulfurous volcano. For more information
about the Io flybys, please visit IoFlyby.com.